This invention relates generally to processor input/output (I/O) interfacing within a computing environment, and more particularly to downbound processing of I/O expansion requests and responses over a PCIe bus and switch architecture.
The Peripheral Component Interconnect Express (PCIe) is a component level interconnect standard that defines a bi-directional communication protocol for transactions between I/O adapters and host systems. PCIe communications are encapsulated in packets according to the PCIe standard for transmission on a PCIe bus. Transactions originating at I/O adapters and ending at host systems are referred to as upbound transactions. Transactions originating at host systems and terminating at I/O adapters are referred to as downbound transactions. The PCIe topology is based on point-to-point unidirectional links that are paired (e.g., one upbound link, one downbound link) to form the PCIe bus. The PCIe standard is maintained and published by the Peripheral Component Interconnect Special Interest Group (PCI-SIG).
One drawback to the use of PCIe is that all I/O adapters connected to a PCIe bus are required to operate within the parameters defined in the PCIe standard (i.e., they are PCIe compatible I/O adapters). The PCIe standard sets rigid constraints on transactions, completion responses and on transaction packaging and addressing. In some system architectures, for example the IBM® System z® architecture, there is a need to be able to support communications over a PCIe bus between I/O adapters and host systems using transactions that are not supported by the PCIe standard. An example is the ability to communicate with non-PCIe compatible adapters (e.g., I/O expansion adapters), which are typically supporting legacy systems and applications that may be difficult (due, for example to technology issues and/or expense) to convert into the PCIe standard. Thus, while PCIe is suitable for its intended purpose of communicating with PCIe compatible adapters, there remains a need for expanding this capability to allow PCIe to communicate with non-PCIe compatible adapters to support legacy systems.